Asynchronous transport mode (“ATM”) is a communications standard that has been defined by certain domestic and international standards organizations for the transport of a broad range of user information, including voice, data, and video, over a user-to-network interface (“UNI”). ATM can be used to aggregate a wide variety of traffic types from a large number of widely dispersed individual users onto a single, high-speed UNI that effectively gives each user access to a “broadband” wide area network (“WAN”), or even a global area network (“GAN”), capable of high-speed voice and/or data transport at rates in accordance with a standardized capacity and quality of service (“QoS”).
In the U.S., a synchronous digital hierarchy (“SDH”) specification is referred to as the “synchronous optical network” (“SONET”) specification, and currently contemplates effective transport rates of from 51.84 megabits per second (Mbps) (“OC-1”), to a maximum of 13.92 gigabits per second (Gbps) (“OC-255”) using an optical fiber medium.
In such a system, users, such as telephone subscribers or data network users, access a high-speed, high-capacity internal, or “core,” transport system through “line units” that interface with the internal transport system, which may be SONET-based, at a switch/multiplexer edge node, or “chassis,” of the system transport core. The interface medium between the users and the line units in the chassis may be copper wire, cable, “fiber” (i.e., optical fiber), or some other physical medium, e.g., air. Regardless of the medium and the transport mechanism, however, it is desirable to provide some form of “survivability,” or “protection” mechanism, for the traffic carried throughout the system, both “internally,” i.e., within the transport system itself, and “externally,” i.e. within the UNIs.